1. Field of the Invention
The present invention relates to glass sheet tempering apparatus and particularly relates to the tempering of large glass sheets, especially those that are shaped prior to being tempered. When glass sheets are tempered, each glass sheet in turn is heated above its annealing range and then rapidly cooled to set the surfaces of the glass sheet while the center is still hot. This action results in the sheet having its surfaces stressed in compression while the intermediate portion is stressed in tension.
The stress pattern imparted to temper glass results in a much stronger sheet than untempered glass, because the glass surfaces, by virtue of being stressed in compression, are much more able to withstand external forces than untempered glass sheets which are not provided with such large compression stresses in the surface area. Moreover, when the outer surface of the glass is penetrated, tempered glass breaks up into small, relatively harmless, smoothly surfaced particles. In contrast, annealed glass fractures more readily, and when fractured, breaks into relatively dangerous, large, jagged fragments.
The uniformity of the size of the shattered particles indicates the uniformity of temper of the glass. The smaller, smoother particles of shattered tempered glass are much safer than the jagged fragements of untempered glass.
More specifically, in a typical tempering operation, a glass sheet is heated nearly to its softening point and then quickly chilled by uniformly exposing the opposite surfaces of the heated glass sheet to streams of a tempering fluid, such as air, arranged to cool both surfaces uniformly and simultaneously. The fluid is disposed through two opposed, spaced plenum chambers, each provided with a set of nozzles. Each set of nozzles faces a different major surface of the glass sheet.
The prior art considered it a prerequisite to uniform tempering to have an even distribution of the cooling air over the glass surfaces. This is usually accomplished by blasting air through a plurality of identical, uniformly spaced, elongated nozzles extending through apertures in apertured walls of the plenum chambers. The nozzles are either moved orbitally or reciprocated transversely of their length through an amplitude sufficient to insure that each increment of the glass sheet area is swept by at least one of the reciprocating nozzles. The distance between the nozzle orifices and the adjacent sheet surfaces has been kept as uniform as possible in order to strive for the goal of uniform tempering of the glass sheet.
It is necessary to impart relative movement between the nozzles moving in unison relative to the glass sheet to avoid nonuniform cooling of the glass. When the nozzles are not moved relative to the major glass surfaces or vice versa, the tempering medium blasts are directed against fixed locations on the glass. Fixed air blasts cool the fixed locations opposite the blasts rapidly while other locations adjacent to the fixed locations are not cooled as rapidly. Without such relative movement, patterns of iridescence form on the surface of the tempered glass. These patterns of iridescence are very annoying when viewed in reflection or in polarized light.
The glass sheet tempering art has developed many techniques for imparting relative motion between the nozzles that face the opposite surfaces of the glass sheet and the major surfaces of said sheet. Some of these involve linear reciprocation of the nozzles in unison. Others involve linear reciprocating movement of glass sheets past a pair of arrays of fixed opposing nozzles. Others involve applying elliptical or circular orbital movement of nozzles relative to a glass sheet supported at a fixed position.
By providing relative movement of the nozzles relative to the major surfaces of the glass sheet, and by applying the streams of air or other tempering medium through the nozzles by pressure from a common source, prior art tempering apparatus provided substantially uniform tempering for flat glass and gently curved glass of relatively small and intermediate sizes. However, as the size and/or shape of automobile backlights and sidelights become larger and more complicated, it has become more and more difficult to temper glass sheets adequately. It has become necessary to supply air or other tempering medium at a greater rate of flow for larger sizes than for smaller sizes in order to assure that the glass is adequately tempered.
The prior art correlated one of the problems of inadequate tempering of the central portion of large glass sheets and/or those having complicated curvatures with the inability to remove air blasted toward the central portion of the glass sheet as readily as that blasted to the portion outside said central portion so as to enable fresh cool tempering medium to replace the warmed tempering medium. The prior art recognized the correlation of the long escape path from the center to the edge of the glass sheet with inadequate center portion temper and relatively large particle size in the central portion after destruction testing.
According to one proposal to solve this problem, the wall of each plenum chamber facing the central portion of a glass sheet undergoing quenching has a greater proportion per unit area apertured than the remainder of the wall facing the portion of the glass sheet outside the central portion. Such a construction causes a slight pressure gradient in the tempering medium from the central region to the outermost regions of the space within which the glass sheet is supported between the plenum chambers for tempering. This slight pressure gradient results in a continuous outward flow from the central portion of the glass to its margin and helps remove air from the vicinity of the glass sheet surface after the relatively cool air supplied through the apertured wall of the plenum chamber has cooled the heated glass surface and has in turn been heated by heat exchange with the glass.
Providing larger openings in the apertured walls of the plenum chamber in the center portion than in the portions beyond the central portion requires more power to operate compressors or fans that supply cool air to the various nozzles to establish a flow rate providing a given temper level. In view of the increasing cost of energy in recent years, it would be desirable to develop an alternate technique that does not involve the use of so much energy to develop a desired degree of temper.
2. Description of Patents of Interest
U.S. Pat. No. 2,401,442 to Weihs uses a pair of axially movable arrays of nozzles that move alternately toward and away from a glass sheet surface in mutually opposite directions to increase the intensity with which a plurality of air streams impinge on each surface of the glass sheet while at the same time to decrease the intensity with which adjacent air streams impinge upon the surfaces of the glass sheet and alternately increasing and decreasing the intensity of each of the sets of streams periodically during the cooling of the glass sheet. The purpose of this invention is to avoid irridescent spots that result from non-uniform cooling of the glass sheet due to the more rapid cooling of regions facing the cool air blasts compared to other regions intermediate the rapidly cooled regions. All the nozzles in each array are at approximately equal, changing distances from the adjacent glass surface.
U.S. Pat. No. 3,125,430 to Richardson relates to glass sheet tempering apparatus that arranges cold gas supply nozzles in bunches of mutually divergent nozzles for the delivery of tempering medium defining a mean direction of flow in such a way as to permit ready escape of tempering medium that is warmed on approaching the glass surface along paths of escape intermediate the supply paths. This arrangement permits additional tempering medium from the divergent nozzles of adjacent bunches. All the openings at the ends of the nozzles that supply tempering medium are at approximately the same distance from the adjacent glass surface during the application of tempering medium.
U.S. Pat. No. 3,186,815 to Jochim discloses a glass tempering apparatus designed to temper different portions of the glass to different degrees of temper by providing a separate set of nozzles moveable relative to the remaining tempering nozzles in a direction parallel to the thickness of a glass sheet being tempered. The purpose of this invention is to provide different portions of the tempered glass sheet with different properties that are associated with different degrees of temper.
U.S. Pat. No. 3,294,519 to Fickes discloses glass sheet tempering apparatus in which air under pressure is supplied to a pair of opposed plenum chambers and imparted through nozzles having a larger proportion of tempering medium-imparting area per unit cross section area in the central portion compared to that of the portions exterior of the central portion. The purpose of this patent is to increase the flow rate of tempering medium against the central portion of the glass sheet undergoing tempering so as to cause a pressure gradient in the tempering medium parallel to the major surfaces of the glass sheet from the central region to the marginal portion of the glass sheet.
The pressure gradient so established permits tempering medium to escape more readily from the central portion of the glass sheet after it chills the glass surfaces, permitting the application of additional tempering medium, particularly in the central portion of the glass sheet. Establishing the pressure gradient in this manner requires additional power to provide additional flow of cold tempering medium in the central region of the plenum chamber which faces the central portion of the glass sheet. The need for additional power consumption to insure adequate temper in the central portion of large glass sheets leaves something to be desired and it would be desirable for the glass sheet tempering art to develop a way of tempering glass sheets at minimum power consumption while permitting the tempering medium applied to the opposite major surfaces of the glass sheet to escape readily throughout the entire extent including its central portion between the apertured walls of the plenum chamber and the major surfaces of the glass sheet.
U.S. Pat. No. 3,332,761 to Fredley et al. discloses a gas hearth bed having exhaust passages interspersed between modules for delivering fluid to effect heat exchange on application toward a major glass sheet surface. The fluid delivery openings of the modules are arranged in a surface parallel to the surface of a glass sheet conveyed past the gas hearth bed. In this patent, the surface changes gradually from a flat surface to a curved surface.
U.S. Pat. No. 3,353,946 to McMaster discloses a blast head structure for apparatus to temper flat glass sheets. Weir structures separate the pressure supply areas from the exhaust areas to throttle the flow of tempering fluid therebetween.
U.S. Pat. No. 3,455,671 to McMaster discloses a gas support bed having exhaust passages with flared ends disposed within and surrounded by supply passages. The flared ends of the exhaust passages divert fluid flows directed through the supply passages to direct the flows at an angle less than 90 degrees toward the surface of a glass sheet to be heat treated. The diverted fluid flows create static fluid pressure between adjacent inlet passages that force the fluid to turn more than 180 degrees in flowing to the exhaust passages. A bed of fluid of approximately uniform thickness is thus provided between the gas support bed and the adjacent surface of the glass sheet.
U.S. Pat. No. 3,776,712 to Wilde discloses a two-stage cooling apparatus comprising a gaseous support type first stage followed by a rotating roller support having a row of spaced nozzles between each pair of adjacent rolls. Since the glass is subject to fracture during the second stage, the latter is designed with an open structure to allow shattered glass to fall out of the path of travel of the succeeding sheet of glass and to permit ease of clearance of cullet. The use of two cooling stages allows the second cooling stage to operate with cooling tempering medium at a higher pressure than the first cooling stage.